Photochemical recording method by means of hexacyanoferrate(iii)

ABSTRACT

PROCESS OF REPRODUCTION USING A RECORDING LAYER CONTAINING HEXACYANOFERRATE (III) IONS AND A SUBSTANCE PROVIDING IONS OF SELECTED METALS. THE LAYER IS EXPOSED TO AN ELECTROMAGNETIC RADIATION IMAGE REDUCING THE HEXACYANOFERRATE (III) IONS TO HEXACYANOFERRATE (II) ION S WHICH REACT WITH THE METAL IONS TO FORM A PRECIPITATE HAVING A STRONG ATTRACTION FOR A CATIONIC DYE. THE EXPOSED LAYER IS DEVELOPED BY CONTRACTING UNIFORMLY WITH A DEVELOPING SOLUTION CONTAINING THE CATIONIC DYE.

United States Patent US. CI. 96-49 12 Claims ABSTRACT OF THE DISQLOSURE Process of reproduction using a recording layer containing hexacyanoferrateflll) ions and a substance providing ions of selected metals. The layer is exposed to an electromagnetic radiation image reducing the hexacyanoferrate(I II) ions to hexacyanoferrate(II) ions which react with the metal ions to form a precipitate having a strong attraction for a cationic dye. The exposed layer is developed by contracting uniformly with a developing solution containing the cationic dye.

This invention relates to recording and reproducing of information, which is provided in the form of electromagnetic radiation.

The iron ferricyanide system based on the photochemical reduction of hexacyanoferrate(III) ions is well known.

According to the type of iron ions used in the development, a negative or positive blue-print is obtained. So, the development of a recording element containing an image differentiation in the form of non-exposed areas containing hexacyanoferrate(III) ions and exposed areas containing practically exclusively hexacyauofeiratefll) ions by means of iron(III) salts gives an intense blue precipitate with hexacyanoferrate(II) ions on the exposed areas (Prussian blue), whereas by means of iron(II) salts an insoluble blue precipitate of iron(II) hexacyanoferrate(III) (Turnbulls blue) is obtained in the nonexposed areas together with a bluish-white precipitate of iron(II) hexacyanoferrate (II) in the exposed areas.

There has now been found a method for the improved reproduction of dyestufi? prints within a broad spectrum of colours by means of hexacyanoferrate(III) ions as light-sensitive substance, which method comprises the steps of exposing to active electromagnetic radiation 3. light-sensitive element (supported or self-supporting layer) containing hexacyanoferrateflll) ions and ions of the metals selected from the group of zinc, cadmium, mercury, copper, silver, gold and lead, preferably those ions which in water precipitate with hexacyanoferrate (II) ions and developing with a cationic dye.

By active electromagnetic radiation is understood electromagnetic radiation having such a wavelength and energy that it effects in recording materials described in the present invention a photochemical reduction of hexacyanoferrate(I'II) ions to hexacyanoferratefll) ions.

The use of hexacyanoferrate(lll) ions such as are present e.g., in potassium hexacyanoferrate(III) offers the advantage of but weakly coloured recording layers.

By the exposure to active electromagnetic rays the hexacyanoferrate(III) ions are reduced to hexacyanoferrate(-II) ions, which react with said metal ions of zinc, cadmium, mercury, copper, silver, gold and lead and form a sparingly water-soluble compound, which has a strong adsorption power (mordanting effect) for cationic compounds, such as cationic dyes. Due to the mordanting effect the cationic dyes will strongly adhere to the recording layer in the exposed areas so that according to the invention erasure-proof prints in most varying colours can be obtained.

The hexacyanoferrate(III) ions may be applied immediately before the exposure to the recording element containing at least one of the metal ions of the cited type. The storage stability of the solution containing the hexacyanoferrate(III) ions can be increased by the incorporation of traces (10" molar parts in respect of the amount of hexacyanoferrate(III) ions) of stronger oxidants, e.g., dichromate ions. The sensitizing liquid preferably contains hexacyanoferrate(III) ions dissolved in water in a concentration between 0.5 and 5% by weight.

If a binder is present in the recording layer it is preferably water-permeable. With binders having a hydrophilic character, e.g. gelatin, very good results are obtained. Hydrophilic binders may be hardened to some extent. As supports for non self-supporting recording layers all kinds of supports of common use in photographic materials may be applied.

According to a particular embodiment the sensitivity of the recording element is improved by a n-type inorganic photoconductive substance used in combination with the hexacyanoferrateflll) ions.

The hexacyanoferrate(III) ions have to be in working relationship or intimate contact with the photoconductor during the exposure, in other words they are incorporated together or separately into the recording layer before the exposure. The application of the hexacyanoferrate(III) ions may proceed immediately before the exposure, e.g. by dipping the recording layer in an aqueous solution of an alkali metal hexacyanoferrate(III).

Photoconductive substances that are preferably used are inorganic zinc compounds known for their photoconductive properties, e.g. zinc oxide. When use is made of such a photoconductive compound zinc ions can be provided by the photoconductor itself, so that no other compounds providing ions of zinc, cadmium, mercury, copper, silver, gold or lead need to be present. A n-type photoconductor that does not provide such ions is, e.g., titanium(IV) oxide.

The recording layer preferably contains from 0.05 to 40% by weight of a compound providing hexacyanoferrate(III) ions, but a higher content may be present.

The content in the recording layer of the compound or compounds providing the cited metal ions may vary within Wide limits since the photoconductor itself may provide said ions. Thus, the recording layer may contain from to 5% by weight of such compounds. Preferred compounds other than the photoconductive compounds are, e.g., zinc lactate, lead acetate, cadmium sulphate, copper(-II) sulphate, mercury(II) nitrate, silver lactate, and gold(III) chloride.

A solution used for developing the exposed recording element contains a cationic dye and is called hereinafter the developing liquid.

According to a preferred embodiment the developing liquid contains a cationic dye derived from a leucobase. Leucobases for producing such dyes are described e.g., in the UK. patent specification 1,032,264, which document has to be read in con-junction herewith. Preferred cationic dyes are triarylmethane dyes and thiazine dyes.

The cationic dye is preferably used in a concentration of 0.2 to 2% by weight in an aqueous solution which may be acidic, e.g., of a pH between 1 and 7. By the acid contained in the developing liquid ions of the type mentioned above can be set free from metals or metal compounds (e.g. metal oxides, bicarbonates or carbonates) which have been incorporated in pigment form into the recording layer.

The developing liquid may contain a softening or swelling agent for the binder wherein the photoconductive substances have been dispersed. In this way the penetrating power of the cationic dye is improved and consequently the density of the obtained image is increased. As swelling agent for gelatin urea may be used. As swelling agent for hydrophobic binding agents methanol, acetone, dioxan, formamide and dimethylformamide occasionally mixed with water may be applied.

The swelling agent may be applied before or simultaneously with the hexacyanoferrate(III) ions to the developing liquid. Further, said liquids may contain, e.g. 0.5 to by Weight of a substance that lowers the surface tension. In this respect are mentioned higher alkyl sulphates, higher alkyl quaternary ammonium salts, watersoluble fatty acid salts, and water-soluble polyoxyethylene compounds. If the image due to absorption of dyestuff in the unexposed areas has a coloured background it can be made more contrasty by a short rinsing with water.

The recording element used according to the present invention does not necessarily contain a binder and may be prepared by simply soaking a porous paper with a solution containing hexacyanoferrate(III) ions and the said metal ions.

In the case the recording element contains a n-type photoconductor a binding agent has to be used.

When a hydrophobic binding agent is used one has to take care that the photoconductive particles protrude from the surface of the recording layer. Preferably however, a water-permeable binding agent having a hydrophilic character is used. Examples of usable binding agents are, e.g., ureaformaldehyde resins, dimethylol melamineformaldehyde resins, copoly(vinyl acetate/vinyl laurate) (80/20) and hydrophilic colloids such as gelatin. The concentration of the photoconductive substance is preferably in the range of to 90% by weight in respect of the total weight of the recording layer.

As n-type photoconductive substances white photoconductive zinc oxide and titanium(IV) oxide are preferred, which substances can be spectrally sensitized as is known from common electrophotography or can be doped, e.g., with aluminium and/ or chromium ions. Preferably use is made of photoconductive zinc oxide prepared according to the French process, viz, zinc oxide containing traces of zinc metal. The spectral sensitivity of the n-type inorganic photoconductive substances can be increased, e.g., by spectrally sensitizing dyes known for that purpose, e.g. those described in UK. patent specifications 1,020,504 and 1,020,506, and in US. patent applications Ser. Nos. 628,551, 639,905 and 665,219.

In general the photoconductive substances are inherently sensitive to light of a wavelength range of 3650 to 4000 angstroms. The exposure energy necessary to activate the photoconductor varies with the type of photoconductor, its sensitization, the amount of binder and other ingredients present in the recording layer. All kinds of exposure devices may be used but high energy sources, e.g. high pressure mercury vapour bulbs, are preferably used.

The recording technique according to the present invention is particularly suited for the reproduction of graphic originals.

The developed image can be used for multicopying, when e.g. an arylmethane dye is image-wise fixed by mordanting, since such a dye is soluble in common hectographic spirit duplicating liquids, e.g., ethanol. It is to be understood that in that case the exposure is preferably carried out in such way that laterally reversed print is obtained.

The following examples illustrate the present invention.

EXAMPLE 1 A subbed polyethylene terephthalate film support was coated at a rate of 1 kg. per 12 sq. m. with a dispersion prepared by ball-milling the following ingredients for 12 hours:

Ethanol-1000 ccs. Zinc lactate-250 g.

Sodium hexametaphosphate (dispersing agent)-l g. Copoly(vinyl acetate/vinyl laurate) (:20)-55 g.

The resulting material was wetted with a solution comprising:

Potassium hexacyanoferrate(IlI)-2 g. Water-50 ccs. Methanol-75 ccs.

After having been dried, the material was exposed through a positive line transparency, and developed with a 0.4% aqueous solution of methylene blue and subsequently rinsed with water.

In the exposed areas dyestuff was precipitated, whereby a blue negative print was obtained.

EXAMPLE 2 A paper support weighing g. per sq. m. was coated at a rate of 1 kg. per 144 sq. m. with a dispersion prepared by ball-milling the following ingredients for 6 hours:

Copoly(vinyl acetate/vinyl laurate) (80r20)--30 g.

Acetone500 ccs.

Sodium hexametaphosphate l g.

20% aqueous solution of potassium hexacyanoferrate (III)8 ccs.

Photoconductive zinc oxide-70 g.

After having been dried, the material was exposed through a positive line transparency, developed with an aqueous solution containing 0.4% by weight of methylene blue and subsequently rinsed with water,

A blue negative print was obtained.

What we claim is:

1. A photochemical recording and reproducing process comprising the steps of image-wise exposing to active electromagnetic radiation a recording layer containing hexacyanoferrate(III) ions, and a substance that provides ions of the metals selected from the group of zinc, cadmium, mercury, copper, silver, gold and lead, and uniformly contacting the exposed layer with a developing solution containing a cationic dye.

2. A photochemical recording and reproduction process according to claim 1, wherein the recording layer contains a n-type inorganic photoconductive substance.

3. A photochemical recording and reproduction process according to claim 2, wherein the n-type inorganic photoconductive substance is photoconductive zinc oxide.

4. A photochemical recording and reproduction process according to claim 2, wherein the photoconductive substance is titanium(IV) oxide.

5. A photochemical recording and reproduction process according to claim 1, wherein said cationic dye is a triarylmethane dye or a thiazine dye.

6. A photochemical recording and reproduction process according to claim 1, wherein the cationic dye is applied from an aqueous medium having a pH of l to 7.

7. A photochemical recording and reproduction process according to claim 1, wherein the recording layer contains 0.05 to 40% by weight of a compound providing hexacyanoferrate(lll) ions.

8. A photochemical recording and reproduction process according to claim 6, wherein the aqueous medium contains the cationic dye in a concentration of 0.2 to 2% by weight.

9. A process as in claim 7 wherein said recording layer comprises a binding agent containing the metal ion-providing substance, said agent being adapted to undergo softening, and said developing agent includes a softening agent for said binding agent.

10. A process as in claim 1 wherein said recording layer comprises a water-permeable binding agent for said metal ion-providing substance and said developing liquid comprises an aqueous solution of said dye.

11. A process as in claim 1 wherein said recording layer and said developing solution are free of iron ions.

12. A process as in claim 1 wherein said recording layer contains about 0.05-40% by weight of a compound 2,346,872 4/1944 Holden 96-49 providing said hexacyanoferrateflII) ions and about 5- 3,207,603 9/ 1965 Savit 96-75 90% by Weight of said substance providing said metal ions. FOREIGN PATENTS References 5 1,032,264 6/1966 Great Britain 96-92 UNITED STATES PATENTS ,521,509 1924 Dore] 9 MURRAY KATZ, Primary Examiner 2,014,692 9/1935 Murray 9692 M. SOFOCLEOUS, Assistant Examiner 2,154,506 4/1939 Hinman et a1. 9692 2,154,506 4/1939 Hinmanetal 96-92 U.S. C1.X.R.

2,189,264 2/1940 Holden 96-92 96-75, 90, 92 

